Piezoelectric resonator, filter, and electronic communication device

ABSTRACT

A piezoelectric resonator includes a substrate, a vibration unit disposed on the substrate and having a structure in which at least one pair of an upper electrode and a lower electrode opposed to each other, the upper and lower electrodes sandwiching the upper and lower surfaces of an internal thin-film portion including at least one layer of a piezoelectric thin-film, and an external thin-film portion provided under the lower electrode and including at least one layer of a piezoelectric thin-film or a dielectric thin-film, the vibration unit being vibrated in an n-th harmonic (n is an integer of 2 or more), the upper electrode and the lower electrode being provided substantially in the positions of the loops of the n-th harmonic.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a piezoelectric resonatorincluding a vibration unit having a multilayer structure includingthin-films made of piezoelectric and dielectric materials, and moreparticularly, to a piezoelectric resonator which is used in a filter, anoscillator or other suitable apparatus and is operated so as to bethickness-longitudinally vibrated in a VHF band, a UHF band, andultra-high frequency band of which the frequencies are higher than thoseof the these bands. Moreover, the present invention relates to a filterand an electronic communication device including the piezoelectricresonator.

[0003] 2. Description of the Related Art

[0004] In some piezoelectric resonators using thickness-longitudinalvibration, a resonance response is achieved in an ultra-high frequencyband by use of a structure in which a piezoelectric film with a verysmall film thickness is interposed between electrodes, utilizing aninversely proportional relationship between the resonance frequency andthe thickness of the piezoelectric film.

[0005] Of the above-described piezoelectric resonators, some of thethickness-longitudinal vibration type are each provided with a substratehaving a hole passing through the substrate from the front surface tothe back surface, a diaphragm made of an SiO₂ thin film disposed on thesubstrate so as to cover the hole, and a vibration unit including a ZnOthin film interposed between a pair of opposed electrodes on thediaphragm.

[0006] Some of the above-described type piezoelectric resonators have astructure in which a piezoelectric film is sandwiched between a pair ofupper and lower electrodes, and in particular, and have a structure inwhich the loops of a fundamental wave is positioned on the electrodes toeliminate the loss of resonance energy caused on the electrodes, so thatthe resonance characteristics are improved.

[0007] The loss of the resonance energy is reduced since the fundamentalwave is used. However, in use of the fundamental wave, the resonancefrequency temperature coefficient is considerably changed, due to thevariation of the ratio of the thicknesses of the SiO₂ thin-film and theZnO thin-film. Therefore, the resonance frequency tends to besignificantly changed with the temperature. Thus, for the piezoelectricresonator, the stability of the resonance frequency to the change oftemperature is low.

SUMMARY OF THE INVENTION

[0008] In order to overcome the problems described above, preferredembodiments of the present invention provide a piezoelectric resonator,a filter, and an electronic communication device, in which the loss ofresonance energy on electrodes is greatly reduced, and the stability ofresonance frequency to the change of temperature is significantlyimproved.

[0009] According to a preferred embodiment of the present invention, apiezoelectric resonator includes a substrate, a vibration unit disposedon the substrate and having a structure in which at least one pair of anupper electrode and a lower electrode are opposed to each other, theupper and lower electrodes sandwiching the upper and lower surfaces ofan internal thin-film portion including at least one layer of apiezoelectric thin-film, and an external thin-film portion providedunder the lower electrode and including at least one layer of apiezoelectric thin-film or a dielectric thin-film, the vibration unitbeing vibrated in an n-th harmonic (where n is an integer of 2 orhigher), the upper electrode and the lower electrode being providedsubstantially in the positions of the loops of the n-th harmonic.

[0010] According to a preferred embodiment of the present invention, theupper electrode and the lower electrode are provided approximately inthe positions of the loops. Thus, the resonance energy on the electrodesis greatly reduced. In addition, the vibration unit is vibrated in thevibration mode of an n-th harmonic. Thus, there is a film-thicknessratio range in which even if the film-thickness ratio of the thin-filmportions is changed to be increased or decreased, the resonancefrequency temperature coefficient is prevented from changingsignificantly. Therefore, the resonance frequency can be stabilizedrelative to the change of temperature by setting the film-thicknessratio to be in this range.

[0011] Preferably, the n-th harmonic is a second harmonic, and the filmthickness ratio r=t_(o)/t_(i) in which t_(o) represents the thickness ofthe external thin-film, and t_(i) represents the thickness of theinternal thin-film is set at a value at which the resonance frequencytemperature coefficient of the entire piezoelectric resonator is nearlyzero.

[0012] In this case, since the n-th harmonic is a second harmonic, theresonance frequency can be more effectively stabilized relative to thechange of temperature.

[0013] More preferably, the thin-film portion that is at least one ofthe internal thin-film portion and the external thin-film portion has acombination in which the respective thin-films have different resonancefrequency temperature coefficients.

[0014] The combination includes any one of the combination of therespective thin-films constituting the internal thin-film portion if theinternal thin-film portion includes a plurality of the thin-films, thecombination of the respective thin-films constituting the externalthin-film portion if the external thin-film portion includes a pluralityof the thin-films, and the combination of the respective thin-filmsconstituting the internal thin-film portion and the external thin-filmportion, respectively.

[0015] Thereby, the resonance frequency temperature coefficient of thepiezoelectric resonator can be more effectively set at zero. Thus, theresonance frequency can be more stabilized relative to the change oftemperature.

[0016] More preferably, the external thin-film portion includes at leastone of a thin-film having a SiO₂ thin-film as a major component, athin-film including an SiN thin-film as a major component, and athin-film including an Al₂O₃ thin-film as a major component.

[0017] More preferably, the internal thin-film portion includes athin-film having ZnO as a major component, a thin-film including AlN asa major component, a thin-film including lead titanate zirconate (PZT)as a major component, a thin-film including lead titanate (PT) as amajor component, and a thin-film including barium titanate (BT) as amajor component.

[0018] More preferably, the substrate has a hole or a concavity, and thevibration unit is disposed above the hole or the concavity. Here, thehole indicates a space extending so as to pass through the substratefrom the front surface to the back surface. The concavity indicates aspace, which is a bottom depression formed in one surface of thesubstrate. The resonance characteristic is greatly improved since thevibration unit is disposed above the hole or the concavity.

[0019] Thus, the piezoelectric resonator of which the resonancefrequency temperature coefficient is small with respect to variations ofa film thickness, that is, the change of the resonance frequency, causedby the change of temperature, is greatly reduced, and the resonanceresponse to the change of temperature is very stable, can be provided.

[0020] Japanese Unexamined Patent Application Publication No.2001-203558 discloses that the resonance frequency temperaturecoefficient of the entire piezoelectric resonator is nearly zero byusing a structure in which a piezoelectric film having a negativeresonance frequency temperature coefficient and a piezoelectric filmhaving a positive resonance frequency temperature coefficient aresandwiched between a pair of upper and lower electrodes, and thereby,the resonance response to the change of temperature is stabilized.

[0021] EP0963040A2 discloses that the resonance energy loss on theelectrodes is eliminated by using the structure in which a piezoelectricfilm is sandwiched between a pair of the upper and lower electrodes, andadopting a configuration in which the loops of a resonance wave arepositioned on the electrodes, so that the resonance characteristic isgreatly improved.

[0022] Japanese Examined Patent Application Publication No. 1-48694discloses that between a pair of piezoelectric films provided between apair of upper and lower electrodes, a thin-film having a resonancefrequency temperature coefficient with a sign different from that of thepiezoelectric films is laminated in the center of the piezoelectricfilms, so that the temperature characteristic of the resonance frequencyis greatly improved.

[0023] However, in such conventional devices, second harmonics are notused, and the loops of a vibration wave are not positioned on theopposite electrodes, in contrast to the present invention. Thus, theeffects of the present invention cannot be achieved.

[0024] The filter in accordance with another preferred embodiment of thepresent invention includes a plurality of the piezoelectric resonatorsaccording to other preferred embodiments of the present invention, theelectrodes in the piezoelectric resonators being connected to theconfiguration of a filter circuit.

[0025] The filter in accordance with another preferred embodiment of thepresent invention includes a plurality of the piezoelectric resonatorsaccording to other preferred embodiments of the present invention, whichare connected in a ladder arrangement.

[0026] The duplexer in accordance with yet another preferred embodimentof the present invention is configured using the filter in accordancewith the above-described preferred embodiments of the present invention.

[0027] The electronic communication device in accordance with a furtherpreferred embodiment of the present invention includes at least onepiezoelectric resonator according to the above-described preferredembodiments of the present invention, the piezoelectric resonator beingused in electronic communication operation.

[0028] Other features, elements, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments thereof with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a cross-sectional view of a piezoelectric resonatoraccording to a preferred embodiment of the present invention;

[0030]FIG. 2 is a graph showing the relationship between thefilm-thickness ratio and the resonance frequency temperature coefficientof the piezoelectric resonator of FIG. 1;

[0031]FIG. 3 is a cross-sectional view of a piezoelectric resonatoraccording to another preferred embodiment of the present invention;

[0032]FIG. 4 is a cross-sectional view of a piezoelectric resonatoraccording to still another preferred embodiment of the presentinvention;

[0033]FIG. 5 is a cross-sectional view of a piezoelectric resonatoraccording to yet another preferred embodiment of the present invention;

[0034]FIG. 6 is a cross-sectional view of a piezoelectric resonatoraccording to another preferred embodiment of the present invention;

[0035]FIG. 7 is a cross-sectional view of a piezoelectric resonatoraccording to still another preferred embodiment of the presentinvention;

[0036]FIG. 8 is a cross-sectional view of a piezoelectric resonatoraccording to yet another preferred embodiment of the present invention;

[0037]FIGS. 9A, 9B, and 9 c are circuit diagrams each showing a filterin which the piezoelectric resonator according to preferred embodimentsof the present invention is used;

[0038]FIG. 10A and 10B are circuit diagrams each showing a filter inwhich the piezoelectric resonator according to preferred embodiments ofthe present invention is used.

[0039]FIG. 11 is a schematic illustration of a duplexer using thepiezoelectric resonator according to preferred embodiments of thepresent invention; and

[0040]FIG. 12 is a cross-sectional view of a piezoelectric resonatoraccording to another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] Hereinafter, preferred embodiments of the present invention willbe described with reference to the drawings in detail.

[0042] The basic structure of a piezoelectric resonator of thispreferred embodiment will be described with reference to FIG. 1.

[0043] Reference numeral 1 designates the entire portion of a resonator.The piezoelectric resonator 1 is preferably a thickness-longitudinalvibration type resonator, and preferably includes a substrate 2, adiaphragm 3, and a vibration unit 4.

[0044] The substrate 2 is preferably made of, e.g., Si (silicon) and hasa hole 5 passing through the substrate 2 from the front surface to theback surface.

[0045] The diaphragm 3 functions as an external thin-film portion and ispreferably made of a SiO₂ (silicon oxide) thin-film. The diaphragm 3 isarranged so as to cover the hole 5.

[0046] The vibration unit 4 is provided on the diaphragm 3, and includesa pair of upper and lower opposed electrodes which define an upperelectrode 4 b and a lower electrode 4 a, and an internal thin-filmportion 4 c having one or more layers. The internal thin-film portion 4c includes at least a piezoelectric film that is interposed between theupper electrode 4 b and the lower electrode 4 a.

[0047] Both of the opposed electrodes 4 a and 4 b are preferably madeof, e.g., Al (aluminum).

[0048] The internal thin-film portion 4 c is preferably made of, e.g., aZnO (zinc oxide) thin-film.

[0049] A production example of the piezoelectric resonator having theabove-described structure will be briefly described. Both surfaces ofthe substrate 2 are heat-oxidized to form heat-oxidized SiO₂ thin-films.The heat-oxidized SiO₂ thin-film on the front surface of the substrate 2constitutes the diaphragm 3. The heat-oxidized SiO₂ thin-film on theback surface of the substrate 2 is patterned correspondingly to the hole5 by photolithography. Thereby, the back surface of the substrate 2 isexposed. The exposed back surface of the substrate 2 is anisotropicallyetched using an alkali solution. This etching reaches the SiO₂ thin-filmon the front surface of the substrate 2, whereby a hole 5 is formed inthe substrate 2. Subsequently, the lower electrode 4 a is formed on thediaphragm 3 of the SiO₂ thin-film on the surface of the substrate 2,preferably by lift-off vapor deposition. Thereafter, the internalthin-film portion 4 c including a ZnO thin-film is formed on the lowerelectrode 4 a and the diaphragm 3 by sputtering or another film-formingtechnique. Subsequently, the upper electrode 4 b is formed on theinternal thin-film portion 4 c by lift-off vapor deposition.

[0050] Thus, the production of the piezoelectric resonator 1 iscompleted.

[0051] In this preferred embodiment, according to the above-describedstructure, the piezoelectric resonator 1 is vibrated in the secondharmonic vibration mode of which the fundamental wave is illustrated bythe broken line in FIG. 1. Moreover, the upper electrode 4 b and thelower electrode 4 a are disposed substantially in the positions of theloops of the second harmonic. The nodes of the second harmonic exist inthe internal and external thin-film portions 3 and 4 c.

[0052] Thus, since the loops of the second harmonic are positioned inthe upper electrode 4 b and the lower electrode 4 a, the loss ofresonance energy on the lower electrode 4 a and the upper electrode 4 bis greatly reduced, and thus, the resonance characteristic issignificantly improved.

[0053] In addition, in this preferred embodiment, the diaphragm 3 andthe respective thin-films of the vibration unit 4 are combined in such amanner that the resonance frequency temperature coefficients thereofhave different signs. Moreover, the thickness ratio r=t_(o)/ t₁ in whicht_(o) represents the thickness of the SiO₂ thin-film as the diaphragm 3,and t_(i) represents the thickness of the ZnO thin-film of the vibrationunit 4 is preferably set at a value at which the resonance frequencytemperature coefficient of the entire piezoelectric resonator 1 becomesnearly zero.

[0054] The thickness ratio r will be further described with reference toFIG. 2.

[0055] In FIG. 2, the film-thickness ratio r is plotted as abscissa, andthe resonance frequency temperature coefficient TCF as the ordinate.Reference numerals (1) and (2) represent a fundamental wave and thesecond harmonic. The thicknesses of the respective thin-films aredesigned in such a manner that the vibration unit 4 of this preferredembodiment is excited in the thickness-longitudinal resonance mode ofthe twice harmonic. The film-thickness ratio r is preferably in therange of about 0.6 to about 1.3.

[0056] When the film-thickness ratio r is in this preferred range, theresonance frequency temperature coefficient TCF is about +10 ppm/° C. toabout −10 ppm/° C. Thus, the resonance frequency temperature coefficientTCF can be set nearly at zero by setting the film-thickness ratio r tobe in the above-described range. Thereby, the vibration frequency of thepiezoelectric resonator 1 can be stabilized relative to the change oftemperature.

[0057] As described above, the resonance frequency temperaturecoefficient can be set nearly at zero by adjustment of thefilm-thickness ratio r. This is due to the external thin-film portion 3made of the SiO₂ thin-film having a positive resonance frequencytemperature coefficient and the internal thin-film portion 4 c made ofthe ZnO thin-film having a negative resonance frequency temperaturecoefficient. Referring to FIG. 2, when the film-thickness ratio t_(o) ofthe external thin-film portion 3 having a positive resonance frequencytemperature coefficient is increased from a value of 1, relatively tothe internal thin-film portion 4 c having a negative resonance frequencytemperature coefficient, so that the film-thickness ratio r becomes morethan 1, the resonance frequency temperature coefficient TCF graduallyapproaches zero until the film-thickness ratio r becomes about 1.3.Moreover, when the film-thickness ratio t_(o) of the external thin-filmportion 3 is further increased to be about 1.3 or more, the resonancefrequency temperature coefficient TCF is further increased from zero. Onthe other hand, when the film-thickness t_(o) of the external thin-filmportion 3 is decreased so that the film-thickness ratio r is reduced,the resonance frequency temperature coefficient TCF gradually approacheszero until the film-thickness ratio r is reduced to about 0.6. When thefilm-thickness to of the external thin-film portion 3 is furtherdecreased to be about 0.6 or smaller, the resonance frequencytemperature coefficient TCF is further increased from zero.

[0058] As described above, the resonance frequency of the piezoelectricresonator 1 can be stabilized relative to the change of temperature byadjustment of the resonance frequency temperature coefficient TCF tosubstantially zero which is carried out by setting of the film-thicknessratio r.

[0059] The present invention is not limited to the above-describedpreferred embodiments. Various applications and modifications cam becarried out without departing from the spirit and the scope of thepresent invention.

[0060] In any of the following preferred embodiments, it is premisedthat a second harmonic is used, and the loops of the second harmonic arepositioned substantially on the upper electrode 4 b and the lowerelectrode 4 a. Accordingly, the range shown in FIG. 2 in which thechange of the resonance frequency with temperature is small can be usedby adjustment of the film-thickness ratio of the external thin-filmportion 3 and the internal thin-film portion 4 c.

[0061] (1) The preferred embodiment shown in FIG. 1 preferably uses asecond harmonic. However, this is not restrictive. The vibration unitmay be vibrated in the vibration mode of an N-th harmonic (n is aninteger of 2 or greater), and the upper electrode 4 b and the lowerelectrode 4 a may be provided in the positions of the loops of the n-thharmonic.

[0062] (2) As shown in FIG. 3, the external thin-film portion 3 may havea two layer structure which includes a heat oxidized SiO₂ film 3 a andan SiN (silicon nitride) film 3 b.

[0063] This external thin-film portion 3 includes two layers havingdifferent resonance frequency temperature coefficients, that is, thefilms 3 a and 3 b. Therefore, the resonance frequency temperaturecoefficient of the entire external thin-film portion 3 can be adjustedby appropriately changing the film-thickness ratio of the films 3 a and3 b. By this adjustment, the temperature-dependent change-ratio of theresonance frequency temperature coefficient of the entire piezoelectricresonator 1 is significantly reduced. Thus, the stability to the changeof temperature is greatly improved.

[0064] (3) As shown in FIG. 4, the external thin-film portion 3 may havetwo layer structure which includes a heat oxidized SiO₂ film 3 c and anSiO₂ film 3 d preferably formed by sputtering.

[0065] According to this structure of the external thin-film portion 3,the temperature characteristic of the resonance frequency of theinternal thin-film portion 4 c can be compensated by adjustment of thetemperature characteristic of the resonance frequency of the entireportions of both of the films 3 c and 3 d constituting the externalthin-film portion 3.

[0066] (4) As shown in FIG. 5, the internal thin-film portion 4 c mayhave a two layer structure including an AlN (aluminum nitride) film 4 c1 and an ZnO film 4 c 2.

[0067] In this structure of the internal thin-film portion 4 c, the AlNfilm 4 c 1 has a positive resonance frequency temperature coefficient,while the ZnO film 4 c 2 has a negative resonance frequency temperaturecoefficient. Therefore, the internal thin-film portion 4 c can beproduced in such a manner that the resonance frequency temperaturecoefficient of the entire piezoelectric resonator 1 approaches zero bycompensating for the resonance frequency temperature coefficient of theexternal thin-film portion 3 made of the heat oxidized SiO₂ utilizingthe two layer structure including the AlN film 4 c 1 and the ZnO film 4c 2. As a result, the resonance frequency temperature coefficient of thepiezoelectric resonator 1 is greatly reduced, so that the temperaturecharacteristic becomes stable.

[0068] (5) As shown in FIG. 6, the external thin-film portion 3 may havethe two layer structure including the heat oxidized SiO₂ film 3 c andthe SiO₂ film 3 d preferably formed by sputtering, and also, theinternal thin-film portion 4 c may have the two-layer structureincluding the AlN film 4 c 1 and the ZnO film 4 c 2. Thereby, the sameoperation and effects as described above can be attained.

[0069] (6) As shown in FIG. 7, the external thin-film portion 3 mayinclude an SiN film, and the internal thin-film portion 4 c may includean AlN film. In this case, the same operation and effects as describedin the Items (3) and (4) above can be attained at the same time.

[0070] (7) As shown in FIG. 8, the external thin-film portion 3 may havea two-layer structure including an AlN film 3 e and an Al₂O₃ (aluminumoxide) film 3 f, and also, the internal thin-film portion 4 c may havean AlN single layer structure. Also, in this case, the same operationand effects as described in Items (3) and (4) above can be attained atthe same time.

[0071] (8) The internal thin-film portion 4 c of the vibration unit mayinclude at least one of a thin-film containing PZT (lead titanatezirconate) as a major component, a thin-film including PT (leadtitanate) as a major component, and a thin-film containing BT (bariumtitanate) as a major component, in addition to the thin-film containingZnO as a major component and the thin-film containing AlN as a majorcomponent.

[0072] (9) The piezoelectric resonator 1 of this preferred embodimentmay be incorporated as a filter element into a π type ladder filtershown in FIG. 9A, an L type ladder filter shown in FIG. 9B, a T typeladder filter shown in FIG. 9C, an L type ladder filter shown in FIG.10A, and an L type ladder filter shown in FIG. 10B. These filters havefilter characteristics that are stable relative to the change oftemperature. For the respective filters, a plurality of theabove-described piezoelectric resonators 1 are provided on the substrate2. The piezoelectric resonators 1 on the substrate 2 can be connectedvia the respective electrodes thereof to provide a filter of which theoperational characteristic is stabilized relative to the environmentaltemperature change.

[0073] (10) The piezoelectric resonator of this preferred embodiment maybe mounted in a portable telephone, a radio-wave LAN, and otherdifferent kinds of communication devices. Thus, when the piezoelectricresonator is used in electronic communication operation of theseelectronic communication devices, the operational characteristic can bestabilized with respect to the environmental temperature-change.

[0074] (11) The piezoelectric resonator 1 of this preferred embodimentmay be used as an element for a duplexer to be mounted onto acommunication device. A duplexer 31 is provided with an antenna-terminal32, a reception side terminal 33, and a transmission side terminal 34 asshown in FIG. 11. The duplexer 31 is configured to include thepiezoelectric resonator of preferred embodiments of the presentinvention or the filter described in Item 9 between the reception sideterminal 33, the transmission side terminal 34 and the antenna terminal32 which permits a high frequency signal in a required frequency band tobe transmitted.

[0075] (12) The structure shown in FIG. 12 may be used as a modificationof the piezoelectric resonator of other preferred embodiments of thepresent invention. In a piezoelectric resonator 51 shown in FIG. 12, aconcavity 53 is formed on the upper surface of a silicon substrate 52. Adiaphragm 54 as the external thin-film potion, preferably including twolayers, that is, a heat-oxidized SiO₂ film 54 a and an SiN film (siliconnitride) 54 b, is arranged so as to cover the upper surface of thesilicon substrate 52 and the concavity 53. A vibration unit 55 isarranged on the diaphragm 54. The vibration unit 55 includes a pair ofupper and lower opposed-electrodes, that is, an upper electrode 58 and alower electrode 56, and an internal thin-film portion 57 including atleast one layer of a piezoelectric film sandwiched between the upperelectrode 58 and the lower electrode 56.

[0076] While preferred embodiments of the invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. A piezoelectric resonator comprising: asubstrate; a vibration unit disposed on the substrate and having astructure in which at least one pair of an upper electrode and a lowerelectrode are opposed to each other, the upper and lower electrodessandwiching the upper and lower surfaces of an internal thin-filmportion including at least one layer of a piezoelectric thin-film; andan external thin-film portion provided under the lower electrode andincluding at least one layer of one of a piezoelectric thin-film and adielectric thin-film; wherein the vibration unit is vibrated in an n-thharmonic, where n is an integer of 2 or greater, the upper electrode andthe lower electrode are located substantially in the positions of theloops of the n-th harmonic.
 2. A piezoelectric resonator according toclaim 1, wherein the n-th harmonic is a second order harmonic, and thefilm thickness ratio r=t_(o)/t_(i) in which t_(o) represents thethickness of the external thin-film, and t_(i) represents the thicknessof the internal thin-film is set at a value at which the resonancefrequency temperature coefficient of the entire piezoelectric resonatoris nearly zero.
 3. A piezoelectric resonator according to claim 1,wherein the respective thin-films of at least one of the internalthin-film portion and the external thin-film portion are combined insuch a manner as to have different resonance frequency temperaturecoefficients.
 4. A piezoelectric resonator according to claim 1, whereinthe external thin-film portion includes at least one of a thin-filmhaving an SiO₂ thin-film as a major component, a thin-film including anSiN thin-film as a major component, and a thin-film including an Al₂O₃thin-film as a major component.
 5. A piezoelectric resonator accordingto claim 1, wherein the internal thin-film portion includes a thin-filmhaving ZnO as a major component, a thin-film including AlN as a majorcomponent, a thin-film including lead titanate zirconate as a majorcomponent, a thin-film including lead titanate as a major component, anda thin-film including barium titanate as a major component.
 6. Apiezoelectric resonator according to claim 1, wherein the substrate hasone of a hole and a concavity, and the vibration unit is disposed abovethe one of the hole and the concavity.
 7. A piezoelectric resonatoraccording to claim 1, wherein the piezoelectric resonator is athickness-longitudinal vibration type resonator.
 8. A piezoelectricresonator according to claim 1, wherein the substrate has a hole thatpasses through a top surface to a bottom surface of the substrate andthe vibration unit is disposed above the hole.
 9. A piezoelectricresonator according to claim 8, further comprising a diaphragm that isarranged to cover the hole.
 10. A piezoelectric resonator according toclaim 1, wherein the opposed electrodes are made of aluminum and theinternal thin-film portion is made of zinc oxide.
 11. A piezoelectricresonator according to claim 1, wherein the n-th harmonic is a secondorder harmonic and nodes of the second harmonic exist in the internaland external thin-film portions.
 12. A piezoelectric resonator accordingto claim 11, wherein the film thickness ratio r=t_(o)/t_(i) in whicht_(o) represents the thickness of the external thin-film, and t_(i)represents the thickness of the internal thin-film is set at a value atwhich the resonance frequency temperature coefficient of the entirepiezoelectric resonator is nearly zero.
 13. A piezoelectric resonatoraccording to claim 12, wherein the film-thickness ratio r is preferablyin the range of about 0.6 to about 1.3.
 14. A piezoelectric resonatoraccording to claim 1, wherein the resonance frequency temperaturecoefficient TCF is about +10 ppm/° C. to about −10 ppm/° C.
 15. Apiezoelectric-resonator according to claim 1, wherein the externalthin-film portion includes two layers having different resonancefrequency temperature coefficients.
 17. A filter including a pluralityof the piezoelectric resonators according to claim 1 and a filtercircuit, wherein the electrodes of the piezoelectric resonators areconnected to the filter circuit.
 18. A filter including a plurality ofthe piezoelectric resonators according to claim 1, wherein thepiezoelectric resonators are connected in a ladder arrangement.
 19. Aduplexer including the filter according to claim
 17. 20. A duplexerincluding the filter according to claim
 18. 21. An electroniccommunication device including at least one piezoelectric resonatoraccording to claim 1.